unify exception handling into Utility.Exception

Removed old extensible-exceptions, only needed for very old ghc.

Made webdav use Utility.Exception, to work after some changes in DAV's
exception handling.

Removed Annex.Exception. Mostly this was trivial, but note that
tryAnnex is replaced with tryNonAsync and catchAnnex replaced with
catchNonAsync. In theory that could be a behavior change, since the former
caught all exceptions, and the latter don't catch async exceptions.

However, in practice, nothing in the Annex monad uses async exceptions.
Grepping for throwTo and killThread only find stuff in the assistant,
which does not seem related.

Command.Add.undo is changed to accept a SomeException, and things
that use it for rollback now catch non-async exceptions, rather than
only IOExceptions.

Remote.hs

@@ -56,7 +56,6 @@ import Data.Ord
 import Common.Annex
 import Types.Remote
 import qualified Annex
-import Annex.Exception
 import Annex.UUID
 import Logs.UUID
 import Logs.Trust
@@ -114,10 +113,10 @@ byNameWithUUID = checkuuid <=< byName
 
 byName' :: RemoteName -> Annex (Either String Remote)
 byName' "" = return $ Left "no remote specified"
-byName' n = handle . filter matching <$> remoteList
+byName' n = go . filter matching <$> remoteList
   where
-	handle [] = Left $ "there is no available git remote named \"" ++ n ++ "\""
-	handle (match:_) = Right match
+	go [] = Left $ "there is no available git remote named \"" ++ n ++ "\""
+	go (match:_) = Right match
 	matching r = n == name r || toUUID n == uuid r
 
 {- Only matches remote name, not UUID -}
@@ -315,8 +314,7 @@ isXMPPRemote remote = Git.repoIsUrl r && "xmpp::" `isPrefixOf` Git.repoLocation
 	r = repo remote
 
 hasKey :: Remote -> Key -> Annex (Either String Bool)
-hasKey r k = either (Left  . show) Right
-	<$> tryNonAsyncAnnex (checkPresent r k)
+hasKey r k = either (Left  . show) Right <$> tryNonAsync (checkPresent r k)
 
 hasKeyCheap :: Remote -> Bool
 hasKeyCheap = checkPresentCheap